"James Hahn" wrote in message
...
Don't pay any attention to replies that talk about converters.

Obviously you don't know what you are talking about.
With such a setup you achieve two things:
1) one battery will always be drainen slightly more than the other.
2) that same battery will alway be charged a little less than the other
because the charger measures the voltage across both batteries in series.

So eventually that battery will be in a continuous state of being empty.
Imagine what happens when you start the enginge or put another heavy load on
a series of batteries where one of them is empty: the empty one will be
charged by the full one, but with reverse polarity.

Have you ever disassembled a broken battery pack of some device? In 99% of
the cases you will find all cells in perfect condition (charged and all)
except one, which was obviously the weakest in the chain, showing a reversed
voltage.

Meindert

"Meindert Sprang" wrote in message
...
snip

Obviously you don't know what you are talking about.
With such a setup you achieve two things:
1) one battery will always be drainen slightly more than the other.
Correct (although it obviously depends on usage).

2) that same battery will alway be charged a little less than the other
because the charger measures the voltage across both batteries in series.
Incorrect. The charger measures the voltage across both batteries in series.
The voltage it sees is the result of both batteries being there, not the one
with the highest or lowest charge. The charger will charge until the
voltage across the two batteries reaches the correct level. Whether
individual cells (and being in the same case or in different cases makes no
difference) took a longer time to get up to par simply doesn't matter.

So eventually that battery will be in a continuous state of being empty.
Imagine what happens when you start the enginge or put another heavy load
on
a series of batteries where one of them is empty: the empty one will be
charged by the full one, but with reverse polarity.
This is not what happens, and I cannot imagine what scenario you have in
mind that could create this situation. Have you ever replaced one battery
in a two-battery 24v system with a brand new battery? What happened when
you started the engine? I have and I can tell you the answer - the
uncharged battery gradually charged, as you would expect.

Have you ever disassembled a broken battery pack of some device? In 99% of
the cases you will find all cells in perfect condition (charged and all)
except one, which was obviously the weakest in the chain, showing a
reversed
voltage.
You have demolished our own argument. All cells were treated identically,
yet one has collapsed! You can't use that evidence to support a claim that
cells treated differently are more liable to collapse (which I assume is
what you are trying to say).

Collapse of one cell is the most common mode of failure for wet cell
batteries, but it is not associated with differential discharge rates.

For years battery manuafacturers have provided a 12v tap across the first
eight cells of high voltage industrial batteries. There has never been any
evidence that using the tap shortens the battery life, and it has never been
the experience of the users.

"James Hahn" wrote in message
...
2) that same battery will alway be charged a little less than the other
because the charger measures the voltage across both batteries in
series.
Incorrect. The charger measures the voltage across both batteries in
series.
The voltage it sees is the result of both batteries being there, not the
one
with the highest or lowest charge. The charger will charge until the
voltage across the two batteries reaches the correct level.

There are two possible scenarios: one battery wears more quickly than the
other, eventually reaching a state where the cell voltages reach higher
values due to increasing internal resistance. Thus the charges shuts off too
early, leaving the better battery not topped up.

The other possibility is that due to a deeper discharge of one battery, it
takes longer for the total voltage to reach the point where the charger goes
from bulk to float. So while for the more empty battery the charge is still
ok (bulk), the other one still gets the full bulk current too while it
should receive a float charge.

Have you ever disassembled a broken battery pack of some device? In 99%
of
the cases you will find all cells in perfect condition (charged and all)
except one, which was obviously the weakest in the chain, showing a
reversed
voltage.
You have demolished our own argument. All cells were treated identically,
yet one has collapsed! You can't use that evidence to support a claim
that
cells treated differently are more liable to collapse (which I assume is
what you are trying to say).

There is always one cell that collapses first (the weakesrt), due to
tolerances in materials, production, etc. By introducing an extra load
(wear) on part of the cells in a string, they are more likely to collapse
first.

Collapse of one cell is the most common mode of failure for wet cell
batteries, but it is not associated with differential discharge rates.

We all know that a lead-acid battery suffers from deep discharge. And in
this setup, it is a plain fact that the battery "below" the tap is
discharged sooner then the one "above" the tap. And if the charger is on
"the outside" terminals, it can only see the charge state of the entire
string, which is in this case, wrong information.

For years battery manuafacturers have provided a 12v tap across the first
eight cells of high voltage industrial batteries. There has never been
any
evidence that using the tap shortens the battery life, and it has never
been
the experience of the users.

Uhm, 12V across 8 cells? You are talking about non-rechargable batteries
here?

Meindert

I've read this thread carefully, and although I'm no sort of expert here, I
think that only Meindert is getting at the truth. Most of the rest of you
are getting confused between voltage and current. Because both batteries are
connected in series, IRRESPECTIVE of the charging mechanism (eg alternator
or smart charger), then both receive an identical charge. If charging starts
from a state where one battery is more discharged than the other, then the
fuller battery will replenish its charge before the other one, and towards
the end of that charging regime its terminal voltage will rise fairly
suddenly from around 13V to around 14.5. Now, depending on the precise
characteristics of the charging source (different for smart chargers and
alternators), the ONLY way that the undercharged battery can continue to
receive charge is to overcharge the now fully charged one. This implies
applying more than 14.5 volts across the full battery, and this will, of
necessity, cause to battery to gas, perhaps even boil in extreme
circumstances. Enough said?

Guy is probably right in saying that small additional current drain from one
of the two batteries is lost in the noise, as it were. But as a concept,
asymmetric current from series-connected batteries is bad news.

Interestingly, one could get round the problem described above by putting a
14.5V zenner diode around the battery which is not being drained by your 12V
circuit, but this isn't without problems either - the diode would have to
be capable of handling the maximum charge current, lets say 30 amps. 30A x
14.5V = 420watts - thats a pretty big zenner on a pretty big heatsink!!!!
HTH

"Meindert Sprang" wrote in message
...
snip

There are two possible scenarios: one battery wears more quickly than the
other, eventually reaching a state where the cell voltages reach higher
values due to increasing internal resistance. Thus the charges shuts off
too
early, leaving the better battery not topped up.
The charger does not monitor cell voltages. It monitors the voltage across
all the cells in series. I don't know what you mean by 'wears', but the
initial charge state of individual cells does not affect the voltage level
at which the charger shuts off.

The other possibility is that due to a deeper discharge of one battery, it
takes longer for the total voltage to reach the point where the charger
goes
from bulk to float. So while for the more empty battery the charge is
still
ok (bulk), the other one still gets the full bulk current too while it
should receive a float charge.
Deeper discharge of some cells in the string will not, of itself, mean that
the charge takes longer. The _difference_ doesn't matter. The amount of
charge required controls the charge time. Whether OP is running his radio
off one battery or both makes no difference to the ah used, and therfore no
difference to the charge time (except, of course, that if he uses a
converter he will be consuming more ah for a given usage than if he uses
just one battery).

Have you ever disassembled a broken battery pack of some device? In 99%
of
the cases you will find all cells in perfect condition (charged and
all)
except one, which was obviously the weakest in the chain, showing a
reversed
voltage.
You have demolished our own argument. All cells were treated
identically,
yet one has collapsed! You can't use that evidence to support a claim
that
cells treated differently are more liable to collapse (which I assume is
what you are trying to say).

There is always one cell that collapses first (the weakesrt), due to
tolerances in materials, production, etc. By introducing an extra load
(wear) on part of the cells in a string, they are more likely to collapse
first.
It's not the extra load that causes cell collapse. Mistreatment of the
batteries (either, or both) will shorten their life. If they are operated as
they are designed to be operated then manufacturing differences between
cells are more significant than the fact that some cells have been
discharged to a lower level than others. If you do a proper study of those
dissambled batteries you will find that the collapsed cell is not random -
it has a strong bias towards the poitive terminal.

Collapse of one cell is the most common mode of failure for wet cell
batteries, but it is not associated with differential discharge rates.

We all know that a lead-acid battery suffers from deep discharge.
Who is talking about deep discharge?

And in
this setup, it is a plain fact that the battery "below" the tap is
discharged sooner then the one "above" the tap. And if the charger is on
"the outside" terminals, it can only see the charge state of the entire
string, which is in this case, wrong information.
It is not "discharged sooner". It is discharged to a level slightly below
the other. The charge state of the 'entire string' is not 'wrong'
information. It will indicate the amout of charge required, and any
differential between individual cells, or sets of cells, is not relevant.

For years battery manuafacturers have provided a 12v tap across the first
eight cells of high voltage industrial batteries. There has never been
any
evidence that using the tap shortens the battery life, and it has never
been
the experience of the users.

Uhm, 12V across 8 cells? You are talking about non-rechargable batteries
here?
No. I am talking about standard industrial lead acid wet cells, so I guess
the number is 10. If you need to know it exactly you can take yourself down
to any nearby warehouse and look at the batteries used in a forklift battery
system, and count the number of cells set aside for the 12v tap.

"James Hahn" wrote in message
news
"Meindert Sprang" wrote in message
...
snip


The other possibility is that due to a deeper discharge of one battery,
it
takes longer for the total voltage to reach the point where the charger
goes
from bulk to float. So while for the more empty battery the charge is
still
ok (bulk), the other one still gets the full bulk current too while it
should receive a float charge.
Deeper discharge of some cells in the string will not, of itself, mean
that the charge takes longer. The _difference_ doesn't matter. The amount
of charge required controls the charge time. Whether OP is running his
radio off one battery or both makes no difference to the ah used, and
therfore no difference to the charge time (except, of course, that if he
uses a converter he will be consuming more ah for a given usage than if he
uses just one battery).

Go and read my last post again. The charge doesn't magically share itself
according to need between the two batteries connected in series. Instead,
the charging current supplies equal charge to both batteries. If one
becomes fully charged before the other, then the only way that the partially
charged battery can get more charge is by overcharging the other. That
overcharge is "energy" which cannot be stored in the battery, but has to go
somewhere, and that somewhere is either/or elctrolysing the battery water to
oxygen and hydrogen, and heat. If the discrepancy is small, it probably
doesn't matter, but if not, then it certainly does.

Forget for a moment the issue of connecting a 12 volt load
to a 24 volt system.

Consider only the problem of properly charging two 12 volt
batteries in series with a single 24 volt charger. My
understanding is that unless the two batteries are identical
(goes way beyond same make or capacity) they will not be
charged to the same voltage. Two electrically non-identical
batteries will have different internal resistances (by
assumption)and thus charge to different voltages. As I
mentioned in an earlier post, Ample Power shares this
understanding and offers a 24 volt charger that is in
reality two 12 volt chargers, one connected to each of the
two 12 volt batteries (which are in series, of course).

If my understanding is incorrect, that would mean that a 12
volt marine battery and 8 AAA's in series would all be
properly charged by a 24 volt charger (with current limited
so as not to destroy the AAA's). An attempted reductio ad
absurdum.

My point is simply that with a real-world installation of
two 12 volt batteries in series, the two batteries will
probably measure different voltages. The greater the
differences between the batteries, the greater the potential
problem (no pun).

Without knowing the internal resistances, etc. of the two
batteries, it is not possible to know the precise effect of
placing a different load on one of the batteries. Doing so
would effectively place the added load resistance in
parallel with the internal resistance of the affected
battery. That could make the two batteries either more or
less alike, depending on the values involved.

All of this is hand-waving, of course, because a battery's
internal resistance is really not a constant, but a function
of the battery's chemistry, state-of-charge, etc. It is a
non-linear resistance, in that it varies with current. I
don't believe we miss the mark here by ignoring these
factors, however.

Regards,

Chuck









Pete Styles wrote:
"James Hahn" wrote in message
news
"Meindert Sprang" wrote in message
...

snip

The other possibility is that due to a deeper discharge of one battery,
it
takes longer for the total voltage to reach the point where the charger
goes
from bulk to float. So while for the more empty battery the charge is
still
ok (bulk), the other one still gets the full bulk current too while it
should receive a float charge.

Deeper discharge of some cells in the string will not, of itself, mean
that the charge takes longer. The _difference_ doesn't matter. The amount
of charge required controls the charge time. Whether OP is running his
radio off one battery or both makes no difference to the ah used, and
therfore no difference to the charge time (except, of course, that if he
uses a converter he will be consuming more ah for a given usage than if he
uses just one battery).


Go and read my last post again. The charge doesn't magically share itself
according to need between the two batteries connected in series. Instead,
the charging current supplies equal charge to both batteries. If one
becomes fully charged before the other, then the only way that the partially
charged battery can get more charge is by overcharging the other. That
overcharge is "energy" which cannot be stored in the battery, but has to go
somewhere, and that somewhere is either/or elctrolysing the battery water to
oxygen and hydrogen, and heat. If the discrepancy is small, it probably
doesn't matter, but if not, then it certainly does.

On Sat, 16 Apr 2005 10:12:46 GMT, "Pete Styles"
wrote:


"James Hahn" wrote in message
news
"Meindert Sprang" wrote in message
...
snip


The other possibility is that due to a deeper discharge of one battery,
it
takes longer for the total voltage to reach the point where the charger
goes
from bulk to float. So while for the more empty battery the charge is
still
ok (bulk), the other one still gets the full bulk current too while it
should receive a float charge.
Deeper discharge of some cells in the string will not, of itself, mean
that the charge takes longer. The _difference_ doesn't matter. The amount
of charge required controls the charge time. Whether OP is running his
radio off one battery or both makes no difference to the ah used, and
therfore no difference to the charge time (except, of course, that if he
uses a converter he will be consuming more ah for a given usage than if he
uses just one battery).

Go and read my last post again. The charge doesn't magically share itself
according to need between the two batteries connected in series. Instead,
the charging current supplies equal charge to both batteries. If one
becomes fully charged before the other, then the only way that the partially
charged battery can get more charge is by overcharging the other. That
overcharge is "energy" which cannot be stored in the battery, but has to go
somewhere, and that somewhere is either/or elctrolysing the battery water to
oxygen and hydrogen, and heat. If the discrepancy is small, it probably
doesn't matter, but if not, then it certainly does.


Pete is right on the mark. Reread his prior post. Also go back and
read Bruce's post on the subject. It is bad news to tap a battery
bank.

You can get away with it for a time but it will shorten battery life
because the two batteries do not get charged equally.
As Pete says, "in order to get the battery that is used the most
charged fully you must overcharge the other". There is no way around
that.

Overcharging a battery not only cause water loss but it can warp the
plates in the battery among other things.

Overcharging is the biggest killer of batteries.
Just ask anyone that still uses a feroresonant charger. They will tell
you batteries last only a couple of years.

Do it properly and use the inverter.

regards
Gary

"Gary Schafer" wrote in message ...
On Sat, 16 Apr 2005 10:12:46 GMT, "Pete Styles"
wrote:


"James Hahn" wrote in message
news
"Meindert Sprang" wrote in message
...
snip


The other possibility is that due to a deeper discharge of one battery,
it
takes longer for the total voltage to reach the point where the charger
goes
from bulk to float. So while for the more empty battery the charge is
still
ok (bulk), the other one still gets the full bulk current too while it
should receive a float charge.
Deeper discharge of some cells in the string will not, of itself, mean
that the charge takes longer. The _difference_ doesn't matter. The amount
of charge required controls the charge time. Whether OP is running his
radio off one battery or both makes no difference to the ah used, and
therfore no difference to the charge time (except, of course, that if he
uses a converter he will be consuming more ah for a given usage than if he
uses just one battery).

Go and read my last post again. The charge doesn't magically share itself
according to need between the two batteries connected in series. Instead,
the charging current supplies equal charge to both batteries. If one
becomes fully charged before the other, then the only way that the partially
charged battery can get more charge is by overcharging the other. That
overcharge is "energy" which cannot be stored in the battery, but has to go
somewhere, and that somewhere is either/or elctrolysing the battery water to
oxygen and hydrogen, and heat. If the discrepancy is small, it probably
doesn't matter, but if not, then it certainly does.


Pete is right on the mark. Reread his prior post. Also go back and
read Bruce's post on the subject. It is bad news to tap a battery
bank.

You can get away with it for a time but it will shorten battery life
because the two batteries do not get charged equally.
As Pete says, "in order to get the battery that is used the most
charged fully you must overcharge the other". There is no way around
that.

Overcharging a battery not only cause water loss but it can warp the
plates in the battery among other things.

Overcharging is the biggest killer of batteries.
Just ask anyone that still uses a feroresonant charger. They will tell
you batteries last only a couple of years.

Do it properly and use the inverter.

regards
Gary

I disagree. Two 12v batteries in series to make a 24v battery will self-level
with respect to charge.

They will self level even if one is tapped to run a VHF. No harm will come
to either battery. Voltage can be likened to pressure.

I hope this helps.

CN

"James Hahn" wrote in message
news
"Meindert Sprang" wrote in message
...
snip

There are two possible scenarios: one battery wears more quickly than
the
other, eventually reaching a state where the cell voltages reach higher
values due to increasing internal resistance. Thus the charges shuts off
too
early, leaving the better battery not topped up.
The charger does not monitor cell voltages. It monitors the voltage
across
all the cells in series.

That is exactly what I meant and exactly the reason why it can go wrong. So
I would like to invite you to shoot holes in my following argument:

The charge of a battery is the product of current x time.

Both batteries are in series and one load is connected to the set, operating
at 24V. Another load is connected across only one battery, operating at 12V.
So it is evident that one battery is discharged more than the other.

I start to charge the set in series, so the current through both batteries
is exactly the same. Since one battery is discharged more than the other and
the current throug both is the same, one battery must be charged longer that
the other. Exactly how am I going to achive that with the same current
through both batteries?
One battery will reach the full state before the other but is still being
charged with full current because the other battery hasn't reached the
voltage that corresponds with full charge.

Now, if you can come up with an valid argument why this shouldn't damage one
battery, I bow and take my hat off.

Meindert